Multiple aliases in ARQ destinationInfo

Chris Purvis CPurvis at MADGE.COM
Fri Jan 22 05:32:57 EST 1999

Time for me to wade in on a couple of points here too, I'm afraid...

First let me say what we've implemented so far:
4.      Respond with ACF if at least one alias is registered, starting with
the first one.

Having said that, I'm not certain that's how things should be.  I believe
the "right" answer is none of those Paul suggests.  My starting point,
though, is that at this stage of placing a call, line-hunting is the job of
the gatekeeper, not the endpoint.  If one wants to do line-hunting at the
endpoint, one should be sending a number of ARQs - note that there's no
reason an endpoint can't have multiple ARQs outstanding at the same time, so
this should not add significantly to call setup latency.
Hence what I believe SHOULD happen is that the gatekeeper
1. If it can match anything within its zone, should ensure that different
aliases in the ARQ do not point to different destinations within the zone.
If there is such disagreement, the GK should return ARJ.
2a. Repeat the same policy at any other levels of search that it engages in
(LRQs etc).
or (probably "better" from the point of view of call setup times, although
less deterministic):
2b. If a search procedure needs to be done outside of the zone, use the
first positive reply you get

One use I see for these multiple aliases, is to avoid the abomination of
dial-prefixes for dialing out through gateways.  The first alias should
point to a gateway (probably by name rather than number), subsequent aliases
in a setup message should tell the gateway what to do.  Clever gatekeepers
may want to have policy on what outside lines are permitted to certain
users, and therefore need to see this information at this stage.
Unfortunately nobody seems to use multiple aliases like this...

In answer to a couple of other specific points:

>In practice, could you tell us an example of a widely deployed
>system where you can "dial" multiple destinations (e.g. home phone, work
>phone, cell phone, fax number, emailID) at once and "it" will get you the
>most suitable destination?  I can imagine a smart phonebook/dialer which
>would take the addresses and "dial" one number at a time until it gets
>connected.  But the smarts is in the application, not in the "switch" (in
>our case the GK).
Are you suggesting that if such a thing hasn't been done before it is not a
"good" thing to do?
Incidentally the start of such a thing does exist, at least in the UK.  I
have friends for whom I can dial a single number.  They tell the server for
this number where they are at any given time, and when I dial their
"personal numebr" I talk to the person, whether they happen to be at home
(on their home number), at work (on their work number) or elsewhere (on
their mobile phone).  We have a huge opportunity to improve users'

>Is an EP required to use the exact same alias list in the Setup that it
>in the ARQ? I don't think so. Sounds like it should use the alias list sent
>back in the ACF's destinationInfo field, which would presumably contain
>the alias(es) that resolved into the provided transport address. Good
>though. Needs more thought. What if more than one alias resolved into the
>address and the GK returned all of the resolved aliases? But then, maybe
>trying to over-engineer the problem. Maybe it just makes good sense to the
>users in these situations to provide a single alias, and everything just
>works... :-)
The ACF's destinationInfo field only appears in version 2, but yes this is
certainly what you should use where present (if it's absent, you can't do
anything other than use what you used in the setup message).  Do any
endpoints complain when they see extra, superfluous aliases in setup
messages addressed to them?  I doubt it.  Even gateways should surely use
what they can make sense of  and ignore anything else.  The question mark is
how the destinationInfo, destExtraCallInfo and remoteExtensionAddress fields
in ACF should be mapped to the various fields in the setup message
(CalledPartyNumber, CalledPartySubAddress, destinationAddress,
DestExtraCallInfo, remoteExtensionAddress).  I seem to remember hearing at
some point that they should be mapped into the fields of the same name,
except that the first destinationInfo that is a phone number is moved into
CalledPartyNumber, and CalledPartySubAddress.  I believe if implemented this
is a potential source of incompatibility with TIPHON however, because (again
from memory - no doubt I'll be corrected if I'm wrong!) TIPHON equipment is
instructed to ignore CalledPartyNumber.  Whatever "correct" procedure is,
though, it should be agreed and documented.

>Simple, return the transport address of the first resolved alias. What's
>with that?
Possible non-determinism (see 2b above).

Dr Chris Purvis - Senior Development Engineer, WAVE CC Software
Madge Networks Ltd, Wexham Springs, Framewood Road, Wexham, Berks.  ENGLAND
Phone: +44 1753 661 359  email: cpurvis at

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